Cholangiocytes are simple epithelia, which line the intrahepatic and extrahepatic bile ducts, and participate in the production of bile. Cholangiocytes are the targets of damage during chronic cholestatic liver diseases (cholangiopathies) such as primary biliary cirrhosis, primary sclerosing cholangitis, and cholangiocarcinoma. Cholangiopathies cause morbidity and mortality and are a major public health concern leading to liver transplantation. Cholangiopathies are characterized by cholangiocyte loss requiring the heterogeneous growth of different sized bile ducts for the maintenance of liver function. The elucidation of the intracellular mechanisms regulating the differential proliferative responses of small and large Cholangiocytes to cholestasis and liver injury/toxins will play an important role in the development of therapeutic strategies for the treatment of cholestatic liver diseases. Recently, we have obtained novel preliminary data demonstrating that small and large cholangiocyte proliferation is differentially controlled by calcium- and cAMP-dependent mechanisms via activation and/or inhibition of nuclear factor of activated T cells (NFAT) and cAMP- response-element-binding protein (CREB) signaling, respectively. Based upon our findings, we propose the novel central hypothesis that the differential activation of the transcription factors NFAT, CREB and inducible cAMP early represser (ICER, an inhibitory member of the CREB family), via calcium- (NFAT and ICER) and cAMP-dependent (CREB) signaling mechanisms stimulated by parasympathetic and sympathetic nerves and gastrin regulate the heterogeneous proliferation of small and large Cholangiocytes during cholestasis and liver injury. To test this hypothesis, we have designed three specific aims: (i) neural-dependent activation of the calcium-dependent transcription factor, NFAT, stimulates small cholangiocyte proliferation with activation of secretin receptor (SR) and cystic fibrosis transmembrane regulator (CFTR) expression;(ii) neural- dependent modulation of the cAMP-dependent transcription factor CREB regulates large cholangiocyte proliferation;and (iii) the gastrointestinal hormone gastrin inhibits large cholangiocyte proliferation during cholestasis and cholangiocarcinoma growth via calcium-dependent activation of ICER expression. It is our firm belief that the results of these studies will provide information that will not only allow us to better understand cholangiocyte pathobiology but will also lead to treatments for cholestatic liver diseases.